Specify FLAG_IMMUTABLE when creating PendingIntent

Targeting S+ (version 31 and above) requires that one of
FLAG_IMMUTABLE or FLAG_MUTABLE be specified when creating a
PendingIntent. So this CL uses FLAG_IMMUTABLE to create
the PendingIntent because the flag is recommended in most cases.

Bug: 192103710
Test: manual
Change-Id: I4c9076e10ee6a6227ae9daea1e2f2ad3363671b8